The present invention generally relates to sensors, and more particularly relates to sensor for contactlessly measuring forces which act on the tire of a wheel, especially during driving.
In view of the increasingly dense traffic and larger engines of motor vehicles, controlled intervention in the brakexe2x80x94and enginexe2x80x94system based on measured data of vehicle dynamics has attained greater significance, since errors in the driver""s, driving behavior can be compensated to a certain extent in this way. The forces acting on the vehicle and, in particular, on the tires play a special part in these types of control operations. Whereas it is possible, as such, to obtain the driving dynamics data of a vehicle through the rotary behavior of the wheels of a vehicle, more recent developments strive to measure the forces acting on the tires of the vehicle in order to obtain an input value that is even more suitable for motor vehicle control systems.
Thus, for example, DE-PS 39 37 966 discloses the possibility of measuring the forces acting on a tire on all three coordinates by means of sensors positioned in the tire. This known principle is complicated and prone to failure. Another disadvantage of the prior-art solution is that it is difficult to mount the tires, since the sensor signals from the tire have to be transmitted to an amplifier on the rim and then from the amplifier to a sensor arranged on the rim, from where they are then contactlessly transmitted to the vehicle body. The above-mentioned document makes no statement concerning the type of transmission between the sensors and the amplifier within the tire. If such a transmission inside the tire is handled by means of lines, then mounting the tires becomes difficult because the connecting lines between the sensor and the amplifier inside the tire must not be damaged during the mounting process.
On the other hand, DE-OS 19744611 discloses that the distance of a measuring point on the inner surface of a tire can be measured by a sensor arranged on the rim without contacts in order to determine the air pressure in the tire.
An object of the present invention is to measure, in a simple manner, transverse forces and longitudinal forces acting on the tire during driving, with the above-mentioned advance work of inserting sensors in the tires and the problems involved in mounting the tires being avoided.
The object of the present invention is solved with method for contactlessly detecting the change in position of a measuring point (11) on or in a tire (1) on the basis of a force (7) by means of a sensor (9) acting on the tire (3) of a wheel especially during driving, with the sensor (9) being in a fixed stationary position in relation to the rim (2) of the wheel, wherein a source of radiation is present and by means of the sensor the change in position of the measuring point (11) in the longitudinal and/or transverse direction (y,x) in relation to the running surface of the tire is detected.
Hence, the invention in principle consists of measuring, by means of a sensor arranged on the rim, the change in position of at least one measuring point on the tire in a longitudinal direction and, in particular, also in a transverse direction in relation to the running surface of the tire, thereby determining the forces.
This gives rise to numerous advantages. In principle, it is now possible to contactlessly measure forces in the tire contact area. Since the sensor (or sensors) are not embedded in the tires, the sensor is not lost when the tire is changed. Due to the contactless detection of the change in position of the measuring point, it becomes much easier to mount the tires since there are no connecting cables between the tire and rim. Moreover, the sensor is extensively protected against being damaged by particularly serious tire deformations (driving over edge of sidewalk, flat tire). In addition, the fact that the sensors are not arranged in the running surface of the tire should have a positive effect on the performance of the tire and its load-carrying ability.
In order to mount the sensor in a particularly well protected manner, the present invention teaches fastening the sensor directly to the rim, whereby a particularly stable reference point is obtained. It is not necessary for the sensor to be in direct contact with the rim base. It can also be arranged on a toe connected to the rim, which protrudes radially outwards and, thus, exhibits a smaller distance to the inner surface of the tire. In this way, more precise measurements can be carried out under certain conditions, because the maximum shift angle as viewed from the sensor is increased significantly. Also, the sensitivity of the system can be increased by reducing the distance between the sensor and measuring point, since the contactless transmission distance for the radiation or electromagnetic fields being applied is shortened considerably.
The system of the present invention measures not only in a longitudinal direction (circumferential direction) and transverse direction of the running surface of the tire, but also (essentially) radially in relation to this. On the basis of the deflection of the running surface of the tire in the tire contact area that can be determined in this way, important parameters regarding vehicle dynamics such as, for example, tire pressure, load of the vehicle, strength of the running surface etc., can also be determined.
Basically, the measuring point can be located anywhere on the tire provided it is ensured that the sensor can detect changes in the position of the measuring point in the desired directions. Thus, for example, the measuring point may be arranged on the sidewall of the tire for measuring the circumferential direction. Moreover, it need not be on the surface of the inner surface of the tire, but can also be inserted in the tire, as long as the sensor can definitely measure changes in position.
If the measuring point is positioned on the inner surface of the tire, it can also be designed in a particularly simple manner as a passive transducer which reflects the radiation emitted by the sensor in a suitable way to the sensor. If transversal forces are to be measured, the measuring point should be located below the running surface of the tire, since this is where the greatest movements of the measuring point in a transverse direction occur.
The location information on the measuring point detected by the sensor are particularly accurate when the sensor operates at the same angle-of-rotation position as that of the measuring point of the wheel, so that such disturbance variables as, for example, torsion in both directions due to the tire contact area, vibration knot in the tire, imbalances etc. can be eliminated by forming the difference of the measured values. The angle-of-rotation position can be determined with sufficient accuracy by means of so-called maximum formation. For this purpose, for example using only one single measuring point, the transverse displacement in the x direction and/or the change in distance in the z direction (deflection) is measured at constant time intervals, which, given constant rotational speeds, correspond to equal angle-of-rotation distances.
If the measuring point passes through the tire contact area, a comparably large change in the z and x directions will be detected, and this can be used for determining the maximum displacements and, hence, the angle-of-rotation position of the wheel. It is not necessary for the location coordinates of only one single measuring point to be detected always during one certain angle-of-rotation position only in order to determine the active forces. Rather the same measuring point (or also several measuring points) can be measured one after the other by one single or several sensors in different angle positions. This, for example, may be important if the deflection of the tire contact area is to be related to the distance of the measuring point without deflection or if the transverse displacement of the measuring point in the tire contact area is to be related to the normal position of the measuring point outside the tire contact area. Using only one single measuring point should result in sufficiently accurate measuring results at high speeds (e.g.  greater than 50 km/h). When measuring lower driving speeds and, hence, rotational speeds of the wheel, it is recommended that several sensors be arranged along the circumference of a rim. In this way, several measurements can be taken in the tire contact area during one rotation of the wheel, through which the measuring points of the individual sensors pass.
The measurement is particularly simple when the measuring point is designed as an active, e.g. radiating, transducer. For this purpose, it could be, for example, an optical or acoustic magnetic or electromagnetic radiation or fields. Also a capacitive measurement can be recommended, provided it is ensured that the sensor can detect changes in position in the desired directions. However, an optical source of radiation is preferable, irrespective of whether the radiator is arranged on the sensor or in the measuring point. If the measuring point is a directed optical source of radiation (radiating or reflecting), then the incidence of the directed radiation on a field of optical probes belonging to the sensor may be a possibility of detecting the movement of the measuring point longitudinally and/or transversely to the tire surface. The sensor on the rim may have a matrix of radiation-sensitive probes, which are addressed differently, depending on the position of the measuring point. A measuring point taking the form of an active transducer (piezoelectric element, force transducer), however, can measure directly the force in a certain direction or several forces in different directions and contactlessly transmit these in coded form to the sensor. On the basis of appropriately formed apertures on the sensor, which allow varying quantities of radiation to pass through to the receiver of the sensor depending on the location of the transmitting beam, the change in the position of the measuring point also can be determined.
If the design on the side facing towards the tire is to be kept simple, it is preferable that the measuring point be of a passive design, and the radiation coming from the measuring point is a (reflecting) share of the radiation emitted from the sensor. In this kind of configuration, for example, the location or intensity of the reflected optical beam can be changed by displacing the measuring point in a longitudinal or transverse direction in relation to the running surface. For this purpose, the measuring point can be designed as a reflective surface, with the reflected beam changing its intensity or angle position (as related to the reflective surface) when the reflective surface is displaced sideways because of the curved inner surface of the tire, so that the change of the point of incidence on the sensor can be considered a measure of the displacement of the measuring point in the directions viewed. Yet, the active beam of the sensor also can systematically scan a specified area on the inner side of the tire, comparable to the scanning of a picture tube, and when it falls on the measuring point, it can record the angle position of the scanning beam at that instant, thereby determining the position of the measuring point. Another possibility would be to save the direction of movement of the measuring point and to have the beam retrace the movement of the measuring point in this way.
The change in location of the measuring point allows only indirect conclusions about the forces acting on the tire. If a signal is to be generated that describes the state of the wheel or the vehicle regarding the driving dynamics, i.e. in particular with respect to the changes in movement and forces that occur, then an evaluation device is applied which generates the desired signals characterizing the vehicle state on the basis of the output signals of the sensor. The measuring device, formed primarily by the sensor and evaluation device, can be mounted in an altogether stationary manner, i.e. particularly on the rim proper, corresponding to the combination of features described in claim 9. However, the evaluation device also can be mounted in a stationary position in relation to the wheel axle, e.g. on the cross member. In this way, the sensor is the only element on the rim requiring advance work, because the evaluation device need not be replaced when the rim is changed. However, the evaluation device also can be mounted centrally on the chassis, so that it can handle the output signals of several sensors (e.g. on the four wheels) either simultaneously or subsequently. The output signals of the sensor, depending on the required output values, e.g. for controlling the vehicle dynamics, are processed differently so that the desired values are available at the output of the evaluation device.
A certain difficulty in transmitting the output signals of the sensor or the evaluation device may possibly be leading these signals out through the rim from inside the tire as well as fastening the sensor or evaluation device on the rim. However, since the valve needs a rim opening in any case and, at the same time, provides a connection between the environment of the wheel and the inside of the wheel, it is advantageous to connect the sensor and/or the evaluation device with the valve or at least to arrange the abovementioned assemblies near the valve. In this manner, the output signals can easily be supplied to the environment of the wheel via the valve. Another considerable advantage offered is that the location of the sensor or evaluation device in the area of the rim can be determined unequivocally. This makes it much easier to mount the tire on the rim in a suitable angular position. Mounting the tire at a correct angle in relation to the rim and, consequently, the measuring point in relation to the sensor can be improved by marking the tire to indicate the correct position of mounting in relation to the rim, with the valve bore on the rim being used as an assigned marking on the rim. The correct mounting position of the tire can be ensured also by means of recesses or protrusions on the rim and tire that are allocated to one another, so that the tire can only be mounted on the rim in the correct position.
The invention is particularly suitable for tubeless tires. It can, however, also be used for tubeless tires, with the inner tire surface being replaced by the inner wall of the tube.